Mechanical impact printers are known to the prior art for imprinting a succession of characters on a succession of labels which removably adhere to an elongated strip of label stock backing. In such printers, the elongated strip is moved under tension along a predetermined path, determined in part by a plurality of print stock guides, and past a print station where the characters are successively imprinted. The print station may include a continuously rotating, cylindrical print wheel having located on a circumferential surface thereof a plurality of raised elements representing the characters to be imprinted, and a hammer mechanism including at least one hammer which has a selectively controllable, pivotal movement in a predetermined plane, whereby the hammer during its travel impacts the elongated strip and an interposed ink ribbon against one of the elements of the print wheel, resulting in imprinting of a single character.
In applications wherein it is desired to imprint a plurality of spaced-apart rows of characters on each label, the print wheel has a corresponding plurality of rows of raised elements on its circumferential surface. The hammer mechanism comprises a hammer bank assembly having a plurality of hammers, one for each character row to be imprinted. An example of such a hammer bank assembly known to the prior art is that taught in U.S. Pat. No. 3,983,806, Ishii, issued Oct. 5, 1976. In hammer bank assemblies of this type, each hammer includes a flat body containing an electrical coil. Secured to one end of the body is a head, typically composed of a plastic material, which contains a planar face for impacting a print stock. Secured to the other end of the body is a base, also typically of plastic material, from which extends a pair of electrically-conductive spring members interconnected with respective ends of the electrical coil. The spring members are in turn received in a support which secures the hammer to a mounting structure. First and second magnet assemblies are supported by the mounting structure in proximity to opposing sides of the body. Application of an electrical signal via the spring members to the coil within the body causes the generation of a magnetic flux which interacts with that produced by the first and second magnet assemblies so that the hammer pivots on the spring members to undergo a planar movement to and from the print stock.
When hammer bank assemblies of this type are installed in impact printers of the type described above, the individual hammers experience a very high failure rate in actual use which necessitates frequent hammer replacement and a consequent loss in productivity of the impact printer.
It is therefore an object of this invention to provide an improved hammer bank assembly for impact printers which imprint a plurality of spaced-apart character rows on an elongated strip of print stock.
It is another object of this invention to provide such an improved hammer bank assembly in which the individual hammers have a much lower failure rate than the hammers used in the hammer bank assemblies of the prior art.
An improvement is provided in a hammer bank assembly including a plurality of spaced-apart, parallel hammers, each of the hammers including a substantially planar, hammer body containing an electrical coil, with each hammer body being supported for movement in a predetermined plane, and with the hammer head being attached to and coplanar with each hammer body. The hammer bank assembly further includes a plurality of flat permanent magnets interleaved with a plurality of hammers so that adjacent ones of the permanent magnets oppose but are separated from each hammer body, whereby each of the hammers is caused to move in a predetermined plane upon application of an electrical signal to the electrical coil within the corresponding hammer body and a resultant magnetic interaction with the adjacent permanent magnets. The improvement is particularly directed to hammer guide means for preventing damage to the electrical coils, the hammer guide means being located in proximity to the hammer heads for limiting deflection of each hammer in a direction transverse to the predetermined plane of hammer movement to a predetermined amount less than that which would result in each hammer body contacting either of the adjacent permanent magnets.
In a preferred embodiment, the function of the hammer guide means is provided by a plurality of hammer guide members each defining therein an elongated, substantially rectangular groove. Each of the hammer heads is provided with a substantially rectangular, elongated portion, and each of the grooves is configured so as to be complementary to the elongated portion of each hammer head. The hammer guide members are secured in a spaced-apart manner so that the elongated portion of the head of each of the plurality of hammers is received in a corresponding one of the grooves, the dimension of each groove in a direction transverse to the predetermined plane of hammer movement being larger than a corresponding dimension of the elongated portion but small enough so that the elongated portion contacts the hammer support member upon deflection of the hammer in directions transverse to its predetermined plane of movement before the associated hammer body contacts either of the adjacent permanent magnets.